1. Field of the Invention
The present invention relates to a seat structure, to be in more detail, a seat structure suitably used for a seat of a transportation equipment such as a plain, a train, a ship, a fork lift and an automobile.
2. Description of the Relevant Art
Patent Document 1 (Japanese Patent Application Laid-open No. 2002-177099) discloses a vehicle seat using a three-dimensional net member (solid knitted fabric) which is excellent in cushioning property even in a thin-type as a cushioning layer, for instance. The cushioning property is served by straining respective cushioning layers made of a three-dimensional net member over a back frame and a cushion frame for forming a seat back and a seat cushion to use as a tension structure.
The vehicle seat disclosed in Patent Document 1 is provided with a sufficient cushioning property even in a thin-type as described above, and excellent air permeability, since it uses a three-dimensional net member. However, the vehicle seat disclosed in Patent Document 1 is allowed to have a vibration absorbing function by providing a flat-type supporting member supported by coil springs at the lower portion of the three-dimensional net member on a seat cushion side, and a vibration absorption measure on a seat back side is not considered to be so important. Besides, the three-dimensional net member provided on the seat back is strained with low tension, which sometimes makes an impression that a slight slack exists in the vicinity corresponding to the waist portion of a seated person, causing a problem in appearance.
Meanwhile, especially in a seat for transportation equipment such as an automobile, improvement in impact absorbing function against collision or the like as well as improvement in the above-described vibration absorbing function is always requested to be solved. Though it is an example of a vehicle seat not using a cushioning layer composed of a tension structure in which a three-dimensional net member or the like is strained over a back frame, in Patent Document 2 (Japanese Patent Application No. Hei 11-278128), for instance, disclosed is a vehicle seat, which includes: a pair of cantilever type outer pipes divided into two and each one end being fixed into each side frame; and inner pipes accommodated in the pair of outer pipes, on the lower portion of the back frame, and when impact force beyond a predetermined limit is inputted by collision, the outer pipes are easily bent rightwards, leftwards, and backwards, and at the same time, the inner pipes are slid in the inside of the outer pipes to hold the waist portion of the human body deeply in the seat back so that a slipping-up phenomenon of the human body is prevented.
In order to improve the impact absorbing function, it is desirable to hold a portion from the waist portion to the haunches of the human body (namely, the pelvis portion) deeply in the seat back at the time of collision, as disclosed in Patent Document 2. This is the same as in the case of a seat using a cushioning layer composed of a tension structure such as a three-dimensional net member strained over a back frame. However, in the structure disclosed in Patent Document 2, even though the outer pipes are divided into two, since the inner pipes are housed therein, the stiffness of the pipes perform as an obstacle to backward movement of the human body such as sliding into the seat back. Therefore, it is requested to realize a structure capable of performing backward displacement of the human body more quickly.